1. Field of the Invention
This invention relates generally to communication circuits and, more particularly, to a circuit on a semiconductor chip used in pulse code modulation (PCM) communication systems typically referred to as a PCM Combo Chip.
2. Discussion of the Related Art
A pulse code modulation telephone system can be considered as consisting of two basic paths. The first path is a transmit path in which the two main elements are a band-pass filter in the frequency range of 300 Hertz to 3400 Hertz, and an analog-to-digital (encoder) converter. The second path is the receive path and has a first stage of a digital-to-analog (decoder) converter followed by a low pass filter with a cut-off frequency of 3400 Hertz. In the technology of communication circuits, the combination chip or combo-chip is a name used for an integrated circuit which includes the circuit components for both the transmit path and the receive path.
A problem which can arise in devices implementing the combination chip is a conversion error produced in the transmit path resulting from an inherent DC voltage offset in the band-pass filter output signal. A DC voltage offset in the filter output signal is encoded as information containing signals and are consequently detected as an error signal in the encoded transmitted signal. To minimize the source of this error, systems specifications such as the CCITT and the AT&T specifications, limit the allowable error to values below seven millivolts. However, the seven millivolt figure can be substantially below the errors actually found in devices using the present technology. A need for reducing the error signals below value of the current specifications therefore becomes important for communication systems.
Referring first to FIG. 1, a procedure currently in use for reducing the DC voltage offset in a PCM transmit path is shown. This solution can be described as the integral of the sign bit and provides a cancellation of the system DC voltage offset for the pulse code modulation output signal. An analog input signal is applied to filter 11 only after the initialization phase, but during the initialization phase the analog input signal is internally grounded.
During the initialization phase, the output signal from filter 11 is combined in combination circuit 11a with a signal from integrating circuit 13 and is applied to analog-to-digital converter 12. The output signal of the analog-to-digital converter 12 is the pulse code modulation output signal with the sign of the output signal being applied to an input terminal of the integrating circuit 13. An understanding of how this circuit can operate can be provided by consideration of the circuit signal as a function of time shown in FIG. 2. FIGS. 2b, 2c and 2d illustrate what happens to the circuit of FIG. 1 at certain time intervals thereof after the identification of the presence of an offset voltage shown in FIG. 2a. Because of the presence of a positive signal at the output of the A to D converter 12, a positive sign bit signal is applied to the input of the Integration Circuit 13 which operates, upon receipt of a positive sign bit signal to reduce the Integration Circuit Output Signal (shown in FIG. 2b) one level. Thus, the reduced Integration Output Signal resulting from the application of the positive sign bit signal to the input of the Integration Circuit 13 causes the input signal to the A to D Converter 12 (see FIG. 2b ) to be correspondingly reduced each time that the Integration Output Signal is reduced (see FIG. 2b ). Accordingly, there reaches a point in time when by this technique the Integration Output Signal reaches a low value that causes the received A to D Converter Input Signal to generate a negative sign bit signal (which is shown as a "0" bit signal in FIG. 2d ). This causes the Integration Circuit 13 to generate an increased output voltage which then causes the input to the A to D Converter 12 to get more positive thereby generating an output positive sign bit signal (a "1" bit signal shown in FIG. 2d) which is part of the alternate positive-negative sign bit signals indicating that compensation of the voltage offset from the Filter 11 has been achieved.
Referring to FIG. 3, a prior art circuit for implementing the Integrating Circuit 13 of FIG. 1 is shown. The sign bit signal is applied to a Select Circuit 31 that selects either a positive or a negative reference voltage i.e. +V.sub.R or -V.sub.R and applies this voltage to the output terminal of the Select Circuit 31, which is the Input for the equation shown in FIG. 3. Capacitor C1 which has been discharged to ground by switch 32, is now coupled between the Select Circuit 31 and the inverting terminal of operation amplifier 33. The non-inverting terminal of amplifier 33 is coupled to ground potential. The+V.sub.R or -V.sub.R voltage at the output terminal of Select Circuit 31 generates an output signal to 11a according to the equation shown in FIG. 3. C1 is much smaller than C2 in order to generate the smallest change in the degree of change in the Integration Circuit Output Signal (see FIG. 2b) in order to reduce the effects of a distortion or error signal that is generated on the receipt of the integration output signal at the Filter 11. In this way, the value of the voltage at the output terminal of operational amplifier 33 will gradually approach the value for compensating the offset output signal of Filter 11.
After initialization, an Analog Input Signal is applied to the Filter 11. However, in the prior art circuit of FIGS. 1 and 3, there is a problem in generating a PCM output Signal that is always an accurate representation of the analog Input Signal. For example, if the Analog Input Signal applied to the Filter 11 after initialization is perfectly sinusoidal (where the mean value is zero or the amount of the positive portion of the sinusoidal analog input signal is equal to the negative portion of the sinusoidal analog input signal), then the prior art circuit can generally generate a PCM Output Signal that is a representation of the Analog Input Signal (except for the inherent distortions created by the Integration Circuit 13 during the positive and negative cycle swings of the sinusoidal Analog Input Signal) that is applied to the Filter 11. However, for those situations (such as in some voice type signals) where the Analog Input Signal has either a negative portion (or a positive portion) of its wave form that is longer in time than its corresponding positive (or negative) portion of its wave form, then the distortion that is created by virtue of the operation of the Integration Circuit 13 to generate increased voltage signals for the longer period negative portion of the wave form (or reduced voltage signals for the longer period positive portion of the wave form) causes the PCM Output Signal to be distorted and not be a true representation of the analog Input Signal.
A need has therefore been felt for an improved Pulse Code Modulation circuit that can overcome the above identified problems with the prior PCM circuits. Also, there is a need for providing an improved PCM circuit that can operate during those periods of time when there is not Analog Input Signal.